In recent years, as the packing densities of semiconductor devices are becoming higher, wires of circuits are becoming finer and the distances between the wires are also becoming narrower. It is necessary to planarize the surface of a semiconductor wafer that is a polishing target, and polishing by a polishing apparatus is performed as one method of carrying out such planarization.
A polishing apparatus includes a polishing table for holding a polishing pad for polishing a polishing target, and a top ring for holding a polishing target and pressing the polishing target against the polishing pad. The polishing table and the top ring are each rotationally driven by a driving portion (for example, a motor). A liquid (slurry) that includes a polishing agent is caused to flow on the polishing pad, and the polishing target that is held by the top ring is pushed against the polishing pad to thereby polish the polishing target.
If the polishing of a polishing target by a polishing apparatus is insufficient, a problem arises such as the risk of a short-circuit occurring due to insulation between circuits not being achieved, while if excessive polishing is performed, problems arise such as an increase in resistance values due to a reduction in the cross-sectional area of the wiring, or the wiring itself is completely removed and the circuit itself is not formed. Therefore, it is necessary for a polishing apparatus to detect the optimal polishing endpoint.
As one polishing endpoint detection means, a method is known that detects a change in a polishing frictional force when polishing has transitioned to a substance that is made of a different material. A semiconductor wafer as a polishing target has a laminated structure made of different materials including a semiconductor material, a conductive material and an insulating material, and the coefficient of friction differs between the different material layers. Therefore, the aforementioned method detects a change in the polishing frictional force that is caused by the polishing transitioning to a different material layer. According to this method, a time at which the polishing reaches a different material layer is the endpoint of the polishing.
A polishing apparatus can also detect a polishing endpoint by detecting a change in a polishing frictional force when the polishing surface of the polishing target becomes flat from a state in which the polishing surface was not flat.
A polishing frictional force that arises when polishing a polishing target appears as the driving load of a driving portion. For example, in a case where a driving portion is an electrically-driven motor, a driving load (torque) can be measured as a current that flows to the motor. Therefore, the motor current (torque current) can be detected with a current sensor, and the endpoint of polishing can be detected based on a change in the detected motor current. Examples of such a technique include techniques described in Japanese Patent Application Laid-Open Publication No. 2005-34992 and Japanese Patent Application Laid-Open Publication No. 2001-198813.